1. Field of the Invention
The present invention relates to a silica glass for photolithography, optical members including the glass, an exposure apparatus including the same, and a method for producing the glass. More particularly, it relates to a silica glass used in photolithography techniques together with light in a wavelength region of 400 nm or shorter or, more preferably, 300 nm or shorter, optical members such as lens and mirror including the glass, an exposure apparatus including the glass, and a method for producing the glass.
2. Related Background Art
In recent years, VLSI has been produced with a higher integration and a higher functionality. Particularly, in the field of logical VLSI, a larger system has been mounted on a chip, namely, system-on-chip technique has been in progress. In conjunction with such a trend, there is a demand for finer processability and higher integration on a wafer, such as that made of silicon, which constitutes a substrate for VLSI. In photolithography techniques according to which fine patterns of integrated circuits are exposed to light and transferred onto wafers such as of silicon, exposure apparatuses called stepper are used.
In the case of DRAM, as an example of VLSI, with the advance from LSI to VLSI, as its capacity gradually increases from 1 KB through 256 KB, 1 MB, 4 MB, and 16 MB to 64 MB, the processing line width required for the stepper correspondingly becomes finer from 10 .mu.m through 2 .mu.m, 1 .mu.m, 0.8 .mu.m, and 0.5 .mu.m to 0.3 .mu.m.
Accordingly, it is necessary for a projection lens of the stepper to have a high resolution and a great depth of focus. The resolution and the depth of focus are determined by the wavelength of the light used for exposure and the N.A. (numerical aperture) of the lens.
The angle of the diffracted light becomes greater as the pattern is finer, whereas the diffracted light cannot be captured when the N.A. of the lens becomes greater. Also, the angle of the diffracted light becomes smaller in the same pattern as its exposure wavelength .lambda. is shorter, thereby allowing the N.A. to remain small.
The resolution and the depth of focus are expressed as indicated by the following equations: EQU resolution=k1.multidot..lambda./N.A. EQU depth of focus=k2.multidot..lambda./N.A..sup.2
wherein k1 and k2 are constants of proportionality.
In order to improve the resolution, either the N.A. is increased or .lambda. is shortened. However, as can be seen from the above equations, it is advantageous, in terms of the depth of focus, to shorten .lambda.. In view of these points of view, wavelength of light sources becomes shorter from g-line (436 nm) to i-line (365 nm) and further to KrF excimer laser beam (248 nm) and ArF excimer laser beam (193 nm).
Also, since the optical system loaded in the stepper is constituted by a combination of numerous optical members such as lenses, even when each lens sheet has a small transmission loss, such a loss is multiplied by the number of the lens sheets used, thereby decreasing the amount of light at the irradiated surface. Accordingly, it is necessary for the optical member to have a high degree of transmittance.
Therefore, in the steppers using light in a wavelength region of 400 nm or shorter, optical glass made by a specific method in view of the shortening of wavelength as well as the transmission loss due to the combination of the optical members is used. Also, in the steppers using light in a wavelength region of 300 nm or shorter, it has been proposed to use synthetic silica glass and a fluoride single crystal such as CaF.sub.2 (fluorite).
As a specific method for measuring internal transmittance, for example, a method of measuring transmittance of optical glass is known from JOGIS 17-1982. Here, the internal transmittance is calculated by the following equation: ##EQU1## wherein .tau. is internal transmittance of the glass when its thickness is 10 mm; d is difference in thickness of a sample; and T1 and T2 are spectral transmission factors of the glass having sample thickness values of 3 mm and 10 mm, respectively, including their reflection loss.